Pipelines are typically laid across large bodies of water by welding one segment of pipe at a time on to the pipeline on a surface ship or barge. The pipelines are typically laid air-full to reduce the weight of pipe which must be supported from the water surface. The pipe must bend sufficiently to accommodate being welded at the water surface. Typically, this bending moment does not cause any problem. But occasionally the bending moment will exceed the capacity of the pipe to bend. This can happen, for example, when the surface ship does not maintain the proper position over the laid pipeline. When the pipeline is forced to bend beyond its capacity to bend, it will buckle. When a buckle is initiated in deep water, the hydrostatic pressure from the water may force the buckle to propagate. The buckle will often also result in cracking the pipe, thus permitting air to escape and the pipeline will fill up with water. The pipeline will then be too heavy to be supported from the surface ship. The pipeline will then be dropped to the ocean floor.
To prevent a buckle from destroying a very long length of laid pipeline, buckle arrestors are incorporated along the pipeline. Buckle arrestors are relatively ridged rings around the circumference of the pipe. These rings are sufficiently stiff to stop buckles. They resemble flanges with the pipe welded to each side. Buckle arrestors are typically placed each 400 to 1000 feet.
When a pipeline has buckled and is laying on the ocean floor, the end of the buckle must be located, the pipe must be cut beyond the end of the buckle, the good pipe must be dewatered, and the good pipe must then be lifted to the surface. An apparatus capable of accomplishing this is described in U.S. Pat. No. 4,444,528. This apparatus comprises means to clamp on to a submerged pipeline, a means to cut the pipe and a recovery header which can then be inserted into the open pipeline. After the recovering header is inserted into the pipeline, it is detached from the clamp, and the clamp is then released and removed to the surface. The recovery header is a sleave which fits inside of the severed pipeline with a bullhead which extends out of the severed pipeline to which a lifting cable is attached. The recovery header is attached to the pipeline by a dowel. The dowel passes through holes drilled in the pipeline by a drill which is attached to the clamp. Dowels are placed through the holes in the pipeline and into receptacles within the recovery header, after the recovery header is inserted into the pipe. The recovery header also serves as a stop for a pig which is used to remove water from the pipeline prior to lifting the pipeline to the surface.
This apparatus solves many of the problems associated with the prior art described within Patent '528. But it does so with an extremely complicated and relatively expensive piece of equipment. Failures of pipe-lines are a relatively rare occurrence. This equipment must therefore be purchased and maintained for very infrequent use. Considering the complex nature of the apparatus, the infrequency of its use and the hostile environment in which it is to be used, a high degree of operational reliability is not to he expected. Further, a considerable amount of money must be invested considering the infrequent use.
Another apparatus which can be used to lift a pipeline from the floor of an ocean is an internal clamp. U.S. Pat. No. 4,234,268 discloses an internal clamp used as a recovery header with an apparatus similar to that disclosed in '528. After the pipeline has been severed, the internal clamp is inserted into the end of the pipeline. This clamp is attached to the pipe by packers which expand outward and press against the inside surface of the pipeline. This internal clamp serves the dual purposes of acting as a pig-stop and as a way to attach a lifting means to the severed pipeline. Because frictional forces against the inside of the pipeline are utilized to attach the internal clamp to the pipeline, very large forces against the pipe wall are required. Further, because the pipeline is typically severed by an explosive cutting means, which causes jagged edges and causes the pipe to flare inwards, the internal clamp must fit very loosely into the pipe. Generating sufficient forces against the internal pipe wall with a very loose initial fit is difficult. Therefore, use of an internal clamp will usually require that a cut be made without jagged edges or other pipe end deformation. This, in turn, requires a saw or high pressure water-jet cut, which requires expensive and complex equipment.
Numerous horseshoe-type external pipeline clamps are also known. U.S. Pat. Nos. 3,101,968, 4,097,084, and 604,758 (filed May 31,1898) disclose various horseshoe-type external clamps. As applied to the recovery of a pipeline from the floor of a body of water, these clamps do not include a means to stop a dewatering pig. Another means to dewater the pipe would therefore be required if an external horseshoe-type clamp were utilized. Further, the horseshoe-type clamps do not permit recovery of the pipeline directly over the "stinger" of a pipe-laying vessel. The stinger of a pipe-laying vessel is a support which extends from the vessel and into the water. It provides support for the pipe as it is being layed. It would be preferable to recover a pipeline directly onto the stinger. A clamp having a streamlined profile would enable the clamp and pipeline to be pulled directly onto the stinger, thus facilitating resumption of pipelaying.
It is therefore an object of the present invention to provide a relatively simple pipeline clamp for attaching a lifting means to a severed pipeline on an ocean floor and a method to recover such a pipeline to the surface. It is a further object to provide such a clamp which utilizes buckle arrestors, or similar circumferential ridges to attach the clamp to the pipeline. It is another object to provide a clamp which is of a streamlined profile which allows recovery over the stinger of a pipe-laying vessel.